In a vehicle brake system, a master cylinder generates oil pressure according to a brake pedal operation of a driver, and this oil pressure is distributed between wheel cylinders. U.S. Pat. No. 4,653,815 further provides a motor drive type actuator for every wheel cylinder to control a brake pressure supply to the respective wheel cylinders. The actuator is provided with a shutoff valve which cuts off the master cylinder and wheel cylinder, and a piston which increases the pressure of the wheel cylinder in response to an electric motor.
The actuator operates according to a signal from a control unit. The pressure of the master cylinder is detected by a pressure sensor. The control unit multiplies this pressure by a coefficient of one or more, so as to compute a target pressure, close the shutoff valve and drive the piston so that the wheel cylinders reach the target pressure.
This device can adapt to normal brake operation or to an antiskid brake function which prevents wheel spin merely by changing over the target value. When one of the actuators is found to be defective, braking is performed by opening all shutoff valves and supplying the pressure of the master cylinder directly to the wheel cylinders.
However, when a specific actuator is occurs. For example, if braking is performed while the left front wheel actuator is defective, brake Pressure is first supplied to the other three wheels by the corresponding pistons. As a result, due to difference force between the left and right front wheels, the vehicle starts to turn right. In particular, on the road surface with a low frictional coefficient, three of the wheels lock due to braking and only the left front wheel rotates.
When the shutoff valve is opened in this state, and the pressure of the master cylinder is supplied directly to all four wheels, the left wheel which was rotating also locks, and it may occur at all four wheels lose their grip in the horizontal direction.
In this case, the vehicle may behave differently from what the driver intends.